Knocking from rear/ under car Aston Martin dbs 1968

ALASTAIR.HARVEY1

1968 ASTON MARTIN

100,000 MILES

Hi, a knocking has started suddenly from underneath the car. It is everytime the clutch is engaged and acceleration started. It does not happen when running at any speed. It is loud and can be physically felt. The is engine running perfectly. No noise when idle.